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Would help to isolate problem between hydraulic and vacuum side. Since you have had 3 units, we'll assume for the moment that part is OK.

Does the vacuum portion feel like it is working? Can you pump the brakes until all vacuum gone, put foot on pedal & press normally, holding pedal down and then start engine. If vacuum side is working properly, pedal should go down some more or get easier to push. Then turn engine off and pedal push should be easy for one or two times then get hard again.

The vacuum comes from base of carb at back thru a check valve to booster and reserve tank. That check valve is a metal cylinder like fitting and has been known to fail. If the test above indicates vacuum difficulty, that would be a good place to check first.

The only way to solve this problem is to diagnose it based on some various testing procedures. I outlined one twice above.

Something easier to do before cracking the wheel cylinders as i mentioned above is to remove the vacuum hose from the side of the power unit and plug it. Start the engine and then check to be sure therre is vacuum drawing at the end of the hose. NO vacuum at the hose could cause your symptoms. Could be other things too. There is no ONE SINGLE problem that can cause the symptom u're experienceing. The problem is solved by the process of elemination. Do the easy test FIRST. Checking the vacuum line for vacuum is the easiest i can think of rite now.

Thank you for the reply, my mechanic & I have done all of the above from A_Z there is vacuum to booster, there is very little travel when applying pedal i literally have to stand on pedal to stop car and no lock up at all. all the brake parts have been replaced, one note if the vacuum line is disconnected from the booster there is little to no brakes at all,the only other thing we are going to try is to back way off the brake shoes and see what happens which we will not be able to do until next week. with car running how far down should the pedal go in neutral? what should the brake back off adjustments be? thank you, Peter

" what should the brake back off adjustments be? "

PLease NO NOT take the fololowing as sarcasm. I have to say it this way to make a point:

THis is very basic brake work 101 procedure and covered in the service manual.

The procedure is to adjust the brake shoes out against the drums until there is a slight to moderate (NOT heavy) drag on the wheel as it is turned by hand. Then back off the adjuster 7 notches.

Again, this is a typical standard run of the mill adjustemnt broadly used and extremely fundemental and simple procedure that nearly ALL EACH AND EVERY DRUM type brake system across nearly ALL EACH AND EVERY manufacturer of cars used for nearly 40 consecutive years. That is to say, the brake shoe adjustemnt procedure IS NOT PACKARD SPECIFIC!!!!

I say this because u claim ALL NEW brake parts installed by a mechanic who obviously does not know what he is doing and definately did not read the manual to find out HOW TO DO IT and obviously is NOT qualified to do such work AND OBVIOUSLY has never worked on drum brakes before in his life.

Pedal should move an inch or two before getting hard. With the new BTV assemblies, does it come completely assembled with all the fittings on the end or do you reuse anything. There is a residual valve at the end of booster that could cause a hard pedal if it were damaged but with different units, not likely unless it is being reused.

If the shoes were adjusted too tightly, then there might not be much travel so that would be something to look at

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I concur with PackardV8, if your mechanic doesn't know how to adjust drum brake shoes, I'd be concerned about anything he did. Not throwing stones, just seems a valid question.

Did you try the standard simple basic test of the vacuum system on the BTV? Start the engine, let in run 15 or 30 seconds to insure the vacuum reserve tank is active. Turn off the engine, step on the brake pedal three or more times. The first 2 or 3 times it should drop, perhaps an inch, as you consume the reserve vacuum. Finally after perhaps 3 or 4 applications there will be no reserve vacuum and the pedal should be resistant to almost any movement. Then, keeping your foot on the pedal, start the engine - the pedal should immediately fall away beneath your foot to whatever point is required for the shoes to make contact with the drums.

If that checks out, your problem is downstream of the booster system.

THe 55-56 service manual is availabl HERE at this WEBsite FOR FREE as a PDF download. READ IT!!!!! Study it!!!! Let your mechanic read it. Print it.. U both NEED it!!!

FWIW,
When I first drove my 56 I had the same symptom as you describe, hard pedal, would stand on them to stop. Had BTV rebuilt by Ed Strain, blew out all steel lines, replaced all rubber lines, wheel cylinders and new shoes, some help but still too hard. Took drums and shoes to local brake shoe reline shop and had shoes matched to drums and much improvement in pedal feel but even better feel with discs up front.

Excellent point Rowdy, if the shoes were relined and the drums were cut (or previously cut) and the shoes were not arc-ground (or shimmed if riveted) to the new diameter, you could have VERY poor brake action. Sorry, I didn't go back and read this all from the beginning, but if the shoes were not replaced, they could easily be either glazed or grease saturated and thus have no effective frictional properties.

Though the brakes on my 56 Caribbean (completely stock system, right down to Marshall linings) were adequate, I thought they should be better and relined them (with Kanter lined shoes), results very dramatic - now just a solid touch of the foot on the BTV and all 4 wheels can be easily locked up. Once the wheels lock, it makes no difference how that happened, drums, disc, BTV or not, none of that has any affect on how quickly the car stops - its all a matter between the tires and the road surface. Sorry for stating the obvious.

Thank You again for all your comments, every thing everyone has mentioned has been done including backing of the brakes 7 notches, I have vacuum as stated, with the accept ion of the foot pedal travel which is about 1/2 with engine off and vacuum depleted. start engine and maybe only an additional 1/2" travel for a total of only about 1". the only thing I can think of is I don't know if the hand brake was disconnected prior to the brake adjustment. Thank You again for every ones help.I am becoming very with my Delmor but feel everything has been done according to the book accept for disconnecting the hand brake which will be done next week, will keep everyone posted.